The present invention relates to a method and apparatus for trimming multiple edges of a print product over the course of several processing steps of a processing cycle. The trimming apparatus comprises a stacking device for holding the print products, and a feeding device for transporting the print products from the stacking device to a trimming device. The trimming device includes a pressing die for pressing the print products onto the trimming table, and knives for trimming the print products positioned on the trimming table. The pressing die and the knives have separate drives controlled by a control unit. The processing cycle comprises the following processing steps:                feeding the print products from the stacking device to the trimming device;        pressing the print products onto the trimming table of the trimming device;        trimming the print products while they are positioned on the trimming table of the trimming device;        releasing the pressing die from the trimming table of the trimming device and conveying the print products away from the trimming table.        
Three-way trimmers are known which trim the goods (e.g. stacks of brochures, book blocks, or the like) by pressing the goods against trimming bars. Three-way trimmers of this type can trim a stack (e.g., a stack consisting of at least one book and/or book block) completely and on all three sides while remaining in the same position. The clamped-in good to be trimmed is aligned while on a trimming table and is then trimmed along the head, foot and fore edge. Depending on the design of the three-way trimmer, the sequence of the trimming steps can also be reversed.
Also known are three-way trimmers that trim the fore-edge and the head and foot during separate machine cycles or processing steps. In that case, the good being trimmed must be transported between trimming operations from one trimming station to the next, which has a negative affect on the desired trimming quality.
During the trimming operation, a pressing die presses the good against the trimming table to prevent any movement caused by the resulting cutting forces. To maintain a stable geometry for the good to be trimmed during the trimming operation, it must be ensured that any air which may still be trapped inside the freshly bound good can escape prior to the trimming. Air that is still present in the good during the trimming operation is damaging in several aspects. First, the friction value between pages is reduced because the air acts in the manner of, an air cushion, thus causing the pages to shift relative to each other during the trimming operation. Second, the enclosed air may increase the thickness of the good in certain regions, which can increase the spring-deflection of the good during trimming, causing the upper layers of the good to be cut longer. Both effects reduce the trimming quality.
German reference DE 42 06 329 A, for example, discloses a method for trimming a good which makes use of a pressing device comprising a drive for adjusting pressing elements on the basis of a predetermined characteristic while taking into account the lowest structural mass. The lifting movement of the pressing element can be predetermined in dependence on the operational height for the good to be trimmed. Also provided is a unit for selecting the pressing force and the lifting height. The desired values for the pressing force and the lifting height are input by an operator. However, the disclosed device does not solve the problems caused by enclosed air and the associated reduced trimming quality.
A three-way trimmer is also disclosed in reference EP 0 740 983 A. The disclosed device includes two side-trimming knives and one fore-edge trimming knife. The knives are provided with separate drives operated by digitally controlled servomotors or stepping motors. The movement sequences of the knives are synchronized with the aid of a joint control computer. A pressing die is also provided with a separate drive, which has a digitally controlled servomotor or stepping motor, and is synchronized, thereby resulting in a reduction of the mechanical expenditure and the space required for the coupling elements.
Reference DE 30 11 090 A discloses a trimming apparatus with a mechanically operated pressing device, for which the pressing force can be adjusted hydraulically. The pressing device is positioned automatically, based on the height of the previous good to be trimmed.